Conventionally the fine particle measuring system of this type measures, based on a ball shaped reference particle having a constant refractive index, such as a polystyrene ball or others, a particle diameter corresponding to that of the reference particle using light scattering of laser beam. In measuring particle diameters of fine particles by using light scattering, since the measured values vary depending on refractive indexes of the fine particles, a particle of a constant refractive index, such as a polystyrene ball, is used as a reference. In the following description Particle diameter means a particle diameter corresponding to that of a reference particle.
Fine particles to be measured by using light scattering of laser beam is contained in a sample fluid which flows in a given direction. The sample fluid is in the liquid phase or gaseous phase. In the case the sample fluid is in the liquid phase, it is usual to provide a flow passage by a passage member. In the case where the sample liquid is in the gaseous phase, a passage is not necessarily provided by a flow passage member. The following description is made of a case where the sample fluid is in the liquid phase, and the flow passage is provided by a flow passage member termed flow cell.
To give an example of the conventional fine particle measuring systems, FIG. 1 shows the block diagram of the system of Japanese Patent Laid Open Publication No. 265550/1986.
One laser beam 2 emitted from a laser beam source 1 is so converged by a condenser lens L that it has a given intensity distribution in a scattering region 4. The scattering region 4 is contained in the interior (not shown) of a flow cell which has a transparent portion for the transmission of the laser beam 2. A flow passage 300 defined by the flow cell is usually orthogonal to the laser beam 2. The fine particles contained in a liquid phase sample fluid flow through the flow passage 300 in the flow direction of the sample fluid indicated by the arrow 6.
The fine particles contained in the sample fluid are exposed to the laser beam 2 while passing through the scattering region 4, and disperse in every direction scattered lights in accordance with an intensity of the radiated beam. A part of the scattered lights in a partial region around a light receiving direction 7 are converged on a diaphragm 9 through a light receiving lens 8 and then pass through the aperture of the diaphragm 9 to be received by a light detector 10 disposed immediately behind the diaphragm 9. There the received scattered lights (hereinafter called scattered light outputs) are converted into an electric signal outputs in proportion with an intensities thereof, and their noise components are removed from the electric signal outputs. Then detected light outputs are obtained.
In many cases, the light receiving direction 7 is orthogonal to the direction of the laser beam in the scattering region 4 and to the flow direction 6. The system of FIG. 1 is of the sidewise scattering type. The diaphragm 9 is for defining an area of the light scattering region 4 in which a real image 14 is effectively detected, and the aperture of the diaphragm 9 defines an area of the scattering region 4 in relationship with the direction of the laser beam 2. The scattering region 4 means a region where the scattered lights dispersed by the fine particles which flow thereinto can be effectively received by the light detector 10.
In the conventional system, particle diameters are measured by the magnitude of the above described detected outputs, and based on numbers of measured diameter number of particles are counted. On the other hand, a separate flow meter (not shown), is attached to the flow passage member for measuring a total volume of the sample fluid which flows through a measuring region thereof for a measuring unit time. Using the measured values thus obtained, for example, particle diameter distributions, i.e., densities of particle numbers according to particle diameters, are computed.
Japanese Patent Laid Open Publication No. 160744/1981, for example, describes a fine particle measuring system of the forward scattering type for measuring sizes and concentrations of the fine particles in a fluid (liquid) using light scattering. FIG. 2 shows a section of the optical system of the conventional fine particle measuring system of the forward scattering type with the sample fluid in the liquid phase. A laser beam (measuring light beam) 2 emitted from a laser beam source 1 is adapted to locate its beam spot at the center of a flow passage 300 at the measuring portion of a flow cell 3 and is absorbed by a beam trap 5 disposed behind the flow cell 3. When fine particles contained in a sample liquid flows through the flow passage 300 in the flow cell 3 in a flow direction indicated by the arrow 6 passes the laser beam 2, scattered light are formed in accordance with the known Mie's light scattering theory, and parts of the scattered lights (M.sub.1, M.sub.2 in FIG. 2) are guided to a light receiving window 10' of a light detector 10 by a light receiving lens 8.
The above described conventional art have the following four problems to be solved. A first problem is poor precision of measured particle diameters of fine particles. A second problem is inaccurate measurement of volumes of the measured sample fluids. A third problem is low effective usage of sample fluids. A fourth problem is that the detected scattered lights include noises. These four problems will be explained one by one.
Initially a first problem to be solved by this invention will be explained.
As seen from the known Mie's light scattering theory, the intensity of the scattered light output termed above not only varies with particle diameters, refractive indexes, etc. of fine particles, but also varies in proportion with the intensities of the light beam radiated to the fine particles. In the case where a photomultiplier is used as the light detector 10, the detected light output described above is in proportion with the scattered light output. Accordingly it is necessary in measuring particle diameters based on the magnitudes of the detected light outputs that the intensity distribution of the laser beam 2 is uniform in the scattering region 4. Generally, however, the intensity distribution of the laser beam 2 is not uniform, and in the scattering region 4 the intensity of the laser beam 2 so much decreases especially in the peripheral portion of the laser beam 2 compared with the central portion thereof that in fact particle diameters cannot be measured accurately.
This problem will be elaborated with reference to FIG. 3.
FIG. 3 is a sectional view of FIG. 1 vertical to the scattering region 4 and including the flow direction 6, and shows the optical system for receiving the scattered lights formed by the fine particles passing through the scattering region 4. In FIG. 3, reference letter F.sub.1 represents the flow line of the sample fluid passing the center of the laser beam 2 in the scattering region 4, reference letter F.sub.2 denotes the flow line of the sample fluid passing the periphery of the laser beam 2.
Usually, the distance between the scattering region 4 and the light receiving lens 8 is about 10-20 mm, the numerical aperture of the light receiving lens 8 is about 0.4-0.5, and the spot size of the laser beam 2 in the scattering region 4 is as tiny as about 20 .mu.m. Accordingly the corresponding numerical aperture of the light receiving lens 8 does not substantially differ between the case that fine particles pass along the flow line F.sub.1 and the case that the fine particles pass along the flow line F.sub.2. It may well be considered that the condensing efficiency of the light receiving lens 8 for converging the scattered lights formed by fine particles onto the window 10' of the light detector 10 is substantially equal between the two cases. Thus, a difference in the magnitudes of the outputs from the light detector 10 of fine particles of the same diameter passing the flow lines F.sub.1 and F.sub.2 are substantially in proportion with the intensity of the laser beam 2 on the flow line of fine particles passing the scattering region 4.
FIG. 4 shows the relationships between the positions of the flow lines and the intensity of the scattered light substituted with the relationships between the positions of the flow lines and the magnitude of the detected light output described above. In FIG. 4(a), the detected light outputs along the solid flow line, one dot flow line and two dot flow line correspond respectively to the solid line, one dot line and two dot line outputs in FIG. 4(b). As shown in FIG. 2, in the case that the intensity distribution of the laser beam 2 has the so called Gaussian distribution, the detected light output is the highest when fine particles pass the center of the laser beam 2 (flow line F.sub.1), and lowers toward the peripheral portion thereof (flow line F.sub.2). As described above, the intensity of the laser beam 2 is much lower in the peripheral portion thereof than the center thereof. Naturally this causes the conventional systems to make large measurement errors of particle diameters.
Next a second problem to be solved by this invention will be explained.
In the conventional art, there is provided a flow meter (not shown) in the flow passage member (flow cell) for measuring the total volume of a sample fluid which has flowed through a given measuring region for a measuring time, and based on the thus given measured value, the volume of the sample fluid which has passed the scattered region 4 is estimated. But this measurement needs a separate flow meter, which makes the system larger and expensive. Besides, the volume which has passed the scattering region 4 is estimated indirectly, which makes the resultant value inaccurate. U.S. Pat. No. 4636075 describes that a flow speed of a sample fluid can be determined, based on an interval between the times of measuring fine particles by two separate parallel beams. This is all the U.S. patent describes, and it includes neither the correction of the magnitude of the detected light output nor the measurement of a volume passing the scattering region.
Next a third problem to be solved by this invention will be explained.
In order to measure finer particles it is necessary to contract the spot diameter of the laser beam thereby to obtain a higher intensity of the laser beam. Specifically, when a minimum particle diameter to be detected is set at about 0.2 .mu.m with a spot diameter of about 20 .mu.m, the effective area of the scattering region is about 4.7.times.10.sup.-2 mm.sup.2. When the spot diameter is set about 12 .mu.ms and the intensity of the laser beam is 10 times, the effective area of the scattering region is smaller than about 3.0.times.10.sup.-3 mm.sup.2. The sectional area of the flow passage 300 of the flow cell 3 cannot be made so small in view of its structure and pressure losses. Specifically the sectional area is about 0.8.times.0.8=0.64 mm.sup.2. Thus, the effective area of the scattering region is less than 1/10 a sectional area of the flow passage 300. Consequently the effective usage of a sample fluid becomes so low that the fully effective use cannot be made of the sample fluid.
Next a fourth problem to be solved by this invention will be explained.
Recently the integrity of the semiconductor has become higher and higher. Accompanying the higher integrity, stricter quality conditions are demanded on the pure water and chemical liquids of high purity which are used in the processing of the semiconductor, and it is required to lower a critical particle diameter to be detected by the fine particle measuring system with a liquid phase sample fluid for measuring fine particles contained in a liquid. The fine particle measuring system with a liquid phase sample fluid measures particles based on the scattered light received by the light detector. The scattered output is proportional to, e.g., about 5th power of a particle diameter in the range of 0.1-0.2 micron meters, and thus as a particle diameter decreases, the scattered light output rapidly becomes so lower to be substantially equal to the noise level that it is impossible to detect fine particles.
The noises are made by various causes. To give an example, in the case where a light detector of high sensitivity, such as a photomultiplier, or other, is used, a maximum noise is caused by stray light (the light other than the scattered light) received together with the scattered light. The stray light mainly includes the transparent surface scattered light formed against surfaces 31,32,33,34 of the flow cell 3 in FIG. 2 (the scattered light SC.sub.1 against the surface 31; SC.sub.4, the surface 34, etc.) and the part of the laser beam 2 reflects reciprocally against the surfaces 31-34 of the flow cell 3, then goes out thereof and is not captured by the beam trap 5, when the laser beam 2 passes therethrough.
Then, the former trasparent surface scattered light will be elaborated.
The explanation will be made on the case that the laser beam source 1 is provided by, e.g., a He-Ne laser beam source, the surfaces 31-34 are made of quartz (refractive index: about 1.46), and a sample fluid is super pure water (refractive index: 1.33). A ratio of refractive indexes, i.e., a relative refractive index obtained when the laser beam 2 passes through a first surface 31 and a fourth surface 34 is about 1.46 (=1.46/1) since the outsides of the surfaces 31 and 34 are in contact with the air (refractive index: 1), but the refractive indexes to a second surface 32 and a third surface 33 is about 1.10 (=1.46/1.33).
The intensity of the surface scattered light formed when the laser beam 2 passes through the surfaces 31-34 much depends on the smoothness of the surfaces, but the surfaces at the laser beam transmitting portion of the flow cell 3 of the fine particle measuring system with a liquid phase sample fluid are polished as the usual optical planes to have substantially the same smoothness. Accordingly the intensities of the surface scattered light against the respective surfaces much depend on the relative refractive indexes, and thus the intensity SC.sub.1 and SC.sub.4 of the surface scattered lights against the first and the fourth planes 31 and 34, which have a higher relative refractive indexes, are much higher than those for the second and the third planes 32 and 33. That is, the noise is attributed to the stray light SC.sub.1 and SC.sub.4 formed against the first and the fourth planes.
Next, the latter reflected light reciprocally against the surfaces 31-34 of the flow cell 3 will be elaborated.
In the case where the laser beam source 1 is used, when the surfaces of the flow cell 3 is arranged exactly orthogonal to the laser beam 2, the light reflected against the surfaces 31-34 reverses through a laser beam emitting window and into a laser resonator 1 thereby to disturb the stability of the laser oscillation. In order to preclude this, it is usual to arrange the surfaces of the flow cell 3 not orthogonal to but inclined by some degrees to the laser beam 2.
In the case where the surfaces of the flow cell 3 in FIG. 2 are orthogonal to the flow cell 3, the reciprocally reflected light is included in the plane including the orthogonal direction and the direction of incidence of the laser beams 2. This state is shown in FIG. 5. Unless the surfaces 31-34 of the flow cell 3 are parallel to one another, the reciprocally reflected light is not in the same plane, but since it is possible to prepare a flow cell with the walls thereof arranged substantially parallel to one another, the optical path of the reciprocally reflected light is substantially in the same plane. When the light beam 2" of the reciprocally reflected light deviating much from the right optical paths 2,2' goes outside through the fourth plane 34, the beam 2" becomes a stray light without being captured by the beam trap 5. The stray light is one cause for the noise.
A first object of this invention is to provide a fine particle measuring method and system which enable precise measurement of particle diameters using light scattering of laser beam.
A second object of this invention is to provide a fine particle measuring method and system using light scattering of laser beam which enable precise measurement of particle diameters and direct measurement of a volume of a sample fluid passing a scattering region, even particle diameter distributions, e.g. densities of numbers of particles according to diameters of particles.
A third object of this invention is to provide a fine particle measuring method and system which improve effective usage of a sample fluid in measuring fine particle diameter using light scattering of laser beam.
A fourth object of this invention is to provide a flow cell for use in fine particle measuring systems which enables great reduction of the stray lights generated from the above described surface scattered lights, especially from the scattered lights on the plane of incidence of a laser beam (a first plane), preferably great reduction of the stray lights generated from the scattered lights against the transmitting surface of a laser beam (a fourth plane), and besides prevention of the generation of the stray lights from the reciprocally reflected lights between the surfaces.